Transistor amplifier with degenerative volume control utilizing a unijunction transistor



May 31, 1966 TRANSISTOR AMPLIFIER WITH DEGENERATIVE VOLUME CONTROLUTILIZING A UNIJUNCTION TRANSISTOR Filed June 12 w. M. MQLEAN ETAL3,254,308

2 Sheets-Sheet 1 BASE TO- BASE RESISTANCE, R OHMS P SIGNAL 1 T OUTPUTAMI? W T Q T O I I I l l l l a l2 5 lo 20 50 IE -MILLIAMPERES INVENTORS.WILL/AM M MCLEA/V KENNETH D. SC'HREDEI? ATTORNEY y 1966 w. M. MCLEANETAL 3,254,308

TRANSISTOR AMPLIFIER WITH DEGENERATIVE YOLUME CONTROL UTILIZING AUNIJUNCTION TRANSISTOR 2 Sheets-Sheet 2 Filed June 12, 1963 UnitedStates Patent 3,254 308 TRANSISTOR AMPLIFIER WITH DEGENERATIVE VOLUMECONTROL UTILIZING A UNIJUNC- TION TRANSISTOR William M. McLean,Rochester, N.Y., and Kenneth D.

Schreder, Columbus, Ohio, assignors to General Dynamics Corporation,Rochester, N.Y., a corporation of Delaware Filed June 12, 1963, Ser. No.287,335 4 Claims. (Cl. 33028) This invention relates to amplifiercircuits and is particularly directed to volume control means for theamplifier.

Ideally, all electrodes of a transistor amplifier must have fixedoptimum biases for high gain, low distortion amplification, yet theoutput signal level must be kept relatively constant over a wide rangeof input signal voltages. Volume controls, either manual or automatic,which vary bias voltages from optimum values invariably introduce signaldistortion. Clipping circuits for establishing constant output levelscannot be used in many amplifiers because of the distortion introducedby the clipper.

An object of this invention is to provide improved volume control meansfor amplifiers which overcome the disadvantages of conventional controlcircuits.

A more specific object of this invention is to provide an improvedvolume control circuit for amplifiers which permits optimum bias valuesand minimum distortion over wide ranges of gain control. I

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the specific embodimentdescribed in the following specification and shown in the accompanyingdrawings in which:

FIG. 1 is a schematic circuit diagram of one volume control embodyingthis invention;

FIG. 2 shows the current-resistance characteristic of a unijunctiontransistor employed in the circuits of FIG. 1; and

FIG. 3 shows the input-output signal voltage characteristic of theamplifier of FIG. 1.

The amplifier of FIG. 1 must accept the signal from source regardless ofa wide range of signal voltages and deliver to output terminals 11 thesignal substantially undistorted and, ideally, at a single voltagelevel. The particular amplifier shown comprises transistor 20 andpush-pull transistors 50 and 51 coupled in cascade between the input andoutput circuits of the system. The particular transistors shown are eachof the P-N-P types with the biasing source connected between positiveterminals 40 and ground. The bias of the base, or control electrode, oftransistor 20 is established by the voltage divider resistors 21 and 22.The common emitter circuit of transistor 20 includes current-limitingresistors 23 and 24 bypassed as shown to ground via condenser 25. Theoutput of transistor 20 comprises the impedance Z of transformer 26 withprimary winding connected between the collector, in the configurationshown, and reference ground. The signal voltage across the outputimpedance is sampled, according to this invention, by the potentiometerincluding resistances 27 and 28. It is apparent that the signal voltageat any point in the load or on the potentiometer is in phase oppositionto the input signal voltage. It is also apparent that if a measuredproportion of the sampled signal voltage is applied to the .controlelectrode of the amplifier, the desired negative fwdback may be obtainedfor good distortionless gain control.

According to an important feature of this invention, a variableresistance, with a nonlinear current-resistance characteristic, isconnected in series between the output and input circuits of thetransistor. For this purpose, it

is found that the base-to-base path of the unijunction transistor 29 iswell suited. The two base terminals 29a and 29b are connected betweenthe junction 27a intermediate the ends of potentiometer 27-28 and thecontrol electrode of transistor 20. Blocking condenser 30 is connectedin the feedback circuit to isolate the bias voltages of the transistors20 and 29. Base terminal 2% is biased via resistor 29a. The emitter 290controls the resistance between the base terminals 29a and 29b.

In FIG. 2 is shown the functional relation of the emitter current of theunijunction transistor and the base-tobase resistance of the transistor.It has been found that the base-to-base resistance at zero emittervoltage and current is very high and that as the emitter currentincreases slightly, the base-to-base resistance suddenly drops to a lowvalue, and remains at low resistance values throughout a wide range ofemitter currents' This resistance characteristic with the abrupt knee29k is, fortunately, ideally suited as a series control resistance inthe negative feedback circuit of amplifier 20. That is, when the emittervoltage exceeds a small value, the base-tobase resistance drops to a lowlevel and permits near maximum negative signal feedback. Conveniently,emitter voltage may be obtained from a signal detector for automaticgain control.

'As shown in the input-output characteristic of FIG. 3,

' the output voltage of the amplifier rises sharply to the knee 43 atabout 5 db above zero and then levels throughout a wide range of inputsignals to produce a substantially constant output signal voltage. Inone amplifier employing the circuits of this invention, an input voltagerange of 16 db resulted in an output signal variation of less than .96db.- This gain characteristic is obtained without disturbing the optimumbiases on the amplifier electrodes, without clipping action, and withoutdistortion.

One means, shown in FIG. 1, for obtaining the control current and/orvoltage of the unijunction emitter 290 comprises a circuit for samplingthe output voltage via the tap 44 on transformer 45, lead 46, rectifier47 and direct current amplifier 48. The parameters of the rectifyingcircuit and the gain of the D.C. amplifier are easily selected toestablish the knee 43 at any desired 0 input signal level.

The input-output gain characteristic of FIG. 3 was obtained in theamplifier of FIG. 1 where transistors 20, 50 and 51 were each of thetype commercially available as 2N652A and where the unijunctiontransistor is of the commercially available type known as 2N49l. Withthese semiconductors, the principal components of the circuit of FIG. 1had the following values:

Resistance 21 ohms 8.2K Resistance 22 do 13K Resistance 23 do 1.1KResistance 24 do 68 Resistance 27 ..do 5.1K Resistance 28 do 10Resistance 29c dn 10K Resistance 48a do 1.5K Condenser 2S mierofarads 45Condenser 30 do 4.7

The particular resistance and component values indi cated in FIG. 1 havebeen found to produce the results Patented May 31, 1966 pedanceconnected in the output circuit of said amplifier, a unijunctiontransistor with two base electrodes and an emitter electrode in whichthe base-to-base resistance is a nonlinear function of the emittercurrent, means forbiasing said unijunction transistor so that saidtransistor presents a low base-to-base resistance for emitter currentexceeding a predetermined magnitude, the path of said two bases beingserially coupled for signal frequencies between said load impedance andsaid control electrode of the amplifier, and means coupled to saidoutput circuit for applying a control voltage to said emitter electrodewhich causes said emitter current to exceed said predetermined magnitudewhen the output voltage of said amplifier exceeds a certain level tocontrol the level of signal voltage fed back between said output loadimpedance and said control electrode.

2. A transistor amplifier with a control electrode, an output electrodeand a common electrode, an output impedance connected in the outputcircuit of said transistor, a unijunction transistor with two baseelectrodes and an emitter electrode, the base-to-base resistance of saidunijunction transistor being a nonlinear function of the emittercurrent, means for biasing said unijunction transistor so that saidtransistor presents a low base-to-base resistance for emitter currentexceeding a predetermined magnitude, a degenerative signal feedbackcircuit between the output and input circuits of said amplifier, thebase-to-base path of said unijunction transistor being connected inseries in said feedback circuit and means coupled to said output circuitfor applying a control voltage to said emitter electrode which causessaid emitter current to exceed said predetermined magnitude when theoutput voltage of said amplifier exceeds a certain level.

3. A transistor amplifier having a control electrode, an outputelectrode and a common electrode, a resistor connected in circuit withsaid output electrode, a signal source connected to said controlelectrode, a signal output circuit coupled across said resistor, afeedback circuit coupled between a signal-voltage point in said outputcircuit and said control electrode to degeneratively feed back signalenergy, a unijunction transistor with two base electrodes and an emitterelectrode, said feedback circuit including the base-to-base variableresistance path of said unijunction transistor, means for biasing saidunijunction transistor so that said transistor presents a low 4,base-to-base resistance for emitter current exceeding a predeterminedmagnitude, and a detector circuit responsive to output signal voltagesfor generating a biasing voltage proportional to the signal voltage, andmeans for applying said biasing voltage to the emitter electrode forincreasing said feedback signal energy when the output voltage of saidamplifier exceeds a certain level.

4. An amplifier with a signal input circuit and a signal output circuit,said output circuit having a point at which the signal voltage is out ofphase with respect to the phase of the applied signal in said inputcircuit, a series resistive feedback circuit connected between saidpoint and said input circuit, a unijunction transistor having a pair ofbase electrodes and an emitter electrode, said base electrodes beingconnected in series in said series circuit for controlling the level ofnegative feedback energy and with a control element means for applybiasing voltage to one of said base electrodes to bias said unijunctiontransistor into the low resistance region of its base-to-base resistancecharacteristic when its emitter current exceeds a predeterminedmagnitude, a detector means responsive to signal level in said amplifierfor generating a proportional control voltage, means for applying saidcontrol voltage to said emitter electrode, the resistance of said pathbeing a nonlinear function of said control voltage so that saidresistance drops from a high to a relatively low value in response to arelatively low control voltage and remains substantially at saidrelatively low resistance throughout the remainder of the range ofcontrol voltage.

References Cited by the Examiner UNITED STATES PATENTS 2,930,996 3/1960Chow et al. 30788.5 X 2,997,603 8/1961 Isabeau 328- X 3,015,076. 12/1961Sheehan 330144 X 3,026,485 3/1962 Suran 307-88.5 X 3,163,828 12/1964Fine 33029 X OTHER REFERENCES Article by Suran, Double Base ExpandsDiode Ap plications, Electronics, March 1955, pp. 198-202.

ROY LAKE, Primary Examiner.

R. P. KANANEN, Assistant Examiner.

1. AN AMPLIFIER HAVING A CONTROL ELECTRODE, AN OUTPUT ELECTRODE AND ACOMMON ELECTRODE, AN OUTPUT LOAD IMPEDANCE CONNECTED IN THE OUTPUTCIRCUIT OF SAID AMPLIFIER, A UNIJUNCTION TRANSISTOR WITH TWO BASEELECTRODES AND AN EMITTER ELECTRODE IN WHITH THE BASE-TO-BASE RESISTANCEIS A NONLINEAR FUNCTION OF THE EMITTER CURRENT, MEANS FOR BIASING SAIDUNIJUNCTION TRANSISTOR SO THAT SAID TRANSISTOR PRESENTS A LOWBASE-TO-BASE RESISTANCE FOR EMITTER CURRENT EXCEEDING A PREDETERMINEDMAGNITUDE, THE PATH OF SAID TWO BASES BEING SERIALLY COUPLED FOR SIGNALFREQUENCIES